Enhancing operational stability of OLEDs based on subatomic modified thermally activated delayed fluorescence compounds

The realization of operationally stable blue organic light-emitting diodes is a challenging issue across the field. While device optimization has been a focus to effectively prolong device lifetime, strategies based on molecular engineering of chemical structures, particularly at the subatomic level, remains little. Herein, we explore the effect of targeted deuteration on donor and/or acceptor units of thermally activated delayed fluorescence emitters and investigate the structure-property relationship between intrinsic molecular stability, based on isotopic effect, and device operational stability. We show that the deuteration of the acceptor unit is critical to enhance the photostability of thermally activated delayed fluorescence compounds and hence device lifetime in addition to that of the donor units, which is commonly neglected due to the limited availability and synthetic complexity of deuterated acceptors. Based on these isotopic analogues, we observe a gradual increase in the device operational stability and achieve the long-lifetime time to 90% of the initial luminance of 23.4 h at the luminance of 1000 cd m−2 for thermally activated delayed fluorescence-sensitized organic light-emitting diodes. We anticipate our strategic deuteration approach provides insights and demonstrates the importance on structural modification materials at a subatomic level towards prolonging the device operational stability.

7. It is suggested to change light green lines to a darker color for clarity in the whole manuscript.
Reviewer #3 (Remarks to the Author): Tang and co-workers reported a series of TADF emitters upon systematic deuteration. The authors clearly express their strategy for prolonging the device operational stability in the introduction section. They also investigated the effect of deuteration on photophysical behavior, intrinsic stability, and device operational stability. Overall, this manuscript is well-written and recommended for publication in Nature Communications after a minor revision as follows.
1. In the introduction section, on page 3, line 32, the authors mentioned that the increased PLQY of the near infra-red emitters is due to the lowering of the non-radiative decay rate (knr) and the reorganization energy (). Did the authors investigate the effect of deuteration on knr,  and even the Huang-Rhys factor? 2. Besides deuterium, do the authors think of any isotope of other elements that can be applied in their future studies?
3. On page 7, line 5, regarding the statement "suggesting the need of a higher activation energy (Ea) for C−D bond breaking in the deuterated compounds". In fact, the lowering of the ZPE can be observed in both the reactants and products, and this nearly cancels out for the BDE. However, the effect would be more pronounced for barriers as the transition states often have weak vibrations with low frequencies.
As a result, investigating the transition state and the corresponding activation energy of the C(H, C(D and C(N bond dissociations would help further understand the reason for the OLED stability. Figures S29 and S30. For the S1 state of DPA-BO, the LSOMO is on the DPA moiety, whereas the HSOMO is on the BO moiety. For the T1 state of DPA-BO, the LSOMO is on the BO moiety, while the HSOMO is on the DPA moiety. Could the authors please check if they are correct? 5. On page 7, line 17, the authors mentioned, "the proton abstraction reaction or high molecular adducts formation is probably less favourable". Could the authors describe more specifically the possible pathways of the proton abstraction reaction and the subsequent degradation of the compounds? 6. On page 8, line 17, the authors mentioned "indicating the presence of strong intermolecular interactions in solid state……". What kind are those possible intermolecular interactions do the authors suggest?

Reviewer #1 (Remarks to the Author):
The study conducted by Tang et al. described an intriguing investigation into the efficacy of deuterated thermally activated delayed fluorescence (TADF) materials in enhancing the operational stability of organic light-emitting diodes (OLEDs). The authors comprehensively examine the effect of deuteration on the TADF properties of the compounds. Through independent deuteration of the donor and the acceptor units, the team investigated the correlation between isotopic effect and the corresponding device operational stability. In detail, a series of isotopic analogues were synthesized, namely DPA-BO, d-DPA-BO, DPA-d-BO, and d-DPA-d-BO.
The results found that the compound photostability increased with the degree of deuteration as confirmed by time-dependent UV-vis and PL spectra in the tests of solution, neat film and doped film. Theoretical calculations also supported an increased bond dissociation energy of the C-D bond compared to regular C-H bond was beneficial to the molecule stability. In particular, deuteration of the acceptor, in addition to a deuterated donor, significantly increased the device lifetime by 57% upon using as the photosensitized materials of the emissive layer in the OLED stack. (from 14 h to 22 h).
Overall, this manuscript is well-organized and well-written, which is also timely and pertinent to the field of OLEDs, and the approached developed in this study can benefit the readership of the related fields of deuterated TADF materials and OLEDs. With moderate revisions and supplementary data, I would recommend this manuscript for publication in Nature Communications. Specific comments are as follows.
Response: We thank the reviewer for his/her positive feedback on our manuscript.
Major issues: 1. It was claimed by the author that the OLED stability may be related to the strong C-D bond than that of the C-H bond. In order to further understand the behind reason, the reviewer strongly suggest the authors add additional information by the DFT calculation.
Response: Thank you for your valuable comments on our manuscript. In response to your suggestion regarding the DFT calculation, we would like to kindly point out that we have indeed incorporated this calculation analysis in Table S6, which summarizes the bond dissociation energies (BDEs) of the C-H and C-D bonds as shown below. The BDE of the C-D bond is found to be ca. 0.1 eV larger than that of the C-H bond upon the deuteration of both the DPA and BO moieties. Such energy difference is equivalent to ca. 2 kcal mol -1 , a value that is larger than the chemical accuracy (i.e. 1 kcal mol -1 ), supporting that the C-D bond is significantly stronger than the C-H bond in the tested series of compounds in this study and that eventually enhancing their photo-and electroluminescence performances. While we have successfully computed the BDEs of the C-H and C-D bonds, we acknowledge the potential significance of the role of transition states in the C-H and C-D bond dissociation. In an attempt to optimize the transition states for the C−H (or C−D) bond dissociation, we encountered convergence issues, likewise, the typical case of H2 dissociation, which highlights the limitations of density functional approximations. These limitations can be circumvented by performing calculations on the C-H or C-D bond activation reactions of the compounds by other molecules or radicals present in the system. Such calculations are undergoing in another TADF system and will be reported in due course. Table S6.
Bond dissociation energy (BDE) of the C−H or C−D bonds of the compounds optimized at the M06-2X/6-311++G(d,p) level.
2. I would like to request authors to provide the photostability data for the case of the mCBP host and BN3-mCBP host systems. Then, we can surely compare the PL and EL stabilities with appropriate discussion.
Response: As suggested by the reviewer, we have now obtained the photostability data of BN3 doped in mCBP, and BN3 with DPA-BO doped in mCBP, as well as their corresponding delayed photoluminescence (PL) decay in the solid-state thin films, illustrated in Figure S46 as shown below. We observe an enhanced photostability in the presence of DPA-BO, with the reduced drop in the PL intensity from 62.0% (BN3) to 46.6% (BN3 with DPA-BO). These findings align with the results of our OLED operation test, as presented in Table 3 that summarises the device characteristics of the vacuum-deposited TADF-sensitized OLEDs in the manuscript. Specifically, we observe an increased LT90 from 1.1 h to 12.3 h in device 7 (BN3 doped in mCBP) and device 3 (BN3 with DPA-BO doped in mCBP), respectively, which were fabricated under the same device architecture but different composition in the emissive layer.
We attribute the enhanced stability of BN3 with DPA-BO doped in mCBP, as evidenced by both the PL and electroluminescence (EL) spectra, to the efficient Förster resonance energy transfer (FRET) mechanism. The FRET process is facilitated by the higher singlet (3.01 eV) and triplet energies (2.93 eV) of the DPA-BO series compared to the singlet energy of BN3 (~2.20 eV). The FRET mechanism is illustrated in Figure R1 as shown below. By introducing the TADF materials, DPA-BO, as assistant host or sensitizer for multi-resonance TADF BN3 guest, efficient FRET is achieved, as evidenced by the absence of emission leakage from DPA-BO (Figure S46b (PL) and Figure 6f (EL)).
Moreover, we observe a decrease in the excited state lifetime of BN3 from 224 µs to 196 µs upon the addition of DPA-BO (Figure S46c), indicating an efficient radiative decay from the S1 state of BN3 followed by complete FRET. The reduction in the density of triplet excitons serves to suppress bimolecular triplet-triplet annihilation and triplet-polaron annihilation, which can otherwise lead to undesired exciton fusion processes, such as high-energy polaron formation, ultimately resulting in emitter degradation (Adv. Opt. Mater. 10, 2200665 (2022)). As a result, the addition of DPA-BO into BN3 doped in mCBP exhibits enhanced PL and EL stabilities.     Von represents turn-on voltage.

d)
CIE coordinates are taken at luminance of 100 cd m −2 .

e)
EQEmax represents maximum external quantum efficiency.

g)
CE represents maximum current efficiency.

h)
PE represents maximum power efficiency.

i)
Operational lifetime projected at 1000 cd m -2 .
We have added the following discussion in the revised manuscript on page 13, lines 28-34: "To have a better understanding of these enhancements, we further investigate the photolysis of the solid-state thin films of BN3 (10%) doped in mCBP, and BN3 (10%) with DPA-BO (15%) doped in mCBP ( Figure S46). We observe an enhanced photostability in the presence of DPA-BO, with the reduced drop in the PL intensity from 62.0% (BN3) to 46.6% (BN3 with DPA-BO), as well as a reduced delayed lifetime (from 224.2 μs to 196.3 μs), possibly attributed to the accelerated RISC process." 3. At the end of the manuscript, the author suggests such strategy could be adopted into highly robust multi-resonance TADF systems. This sounds a bit abrupt. Please elaborate how could the approach in this system applied to MRTADF systems, and what could be the potential challenges?
Response: We appreciate the reviewer's feedback regarding the suggestion made at the end of the manuscript. We agree that further elaboration is necessary to clarify how the approach employed in this system could be applied to MRTADF systems.
Narrowband emissive organoboron emitters featuring the multi-resonance (MR) effect have emerged as crucial components for constructing high-performance OLEDs with pure emission colours. These MR organoboron emitters exhibit high-efficiency narrowband TADF through triplet-to-singlet reverse intersystem crossing (RISC). Figure R2 illustrates the molecular structures of DABNA-1 and v-DABNA as examples (Comm. Chem. 5, 149 (2022)). In recent reports, the synthesis procedures of these emitters have employed carbazole, diarylamine or phenol derivatives as building blocks. Our study demonstrates that these starting materials can be readily replaced with deuterated counterparts, opening possibilities for a new class of deuterated MRTADF emitters for OLED applications. Building upon our current study, it is reasonable to expect that the OLED operation stability of these deuterated emitters can be further enhanced. However, achieving meaningful improvements in stability requires precise control over deuteration and careful optimization. The challenges of experimental complexity and scale-up must also be addressed to enable practical implementation. These aspects call for focused efforts in deuteration control and optimization strategies to fully harness the potential of deuterated MRTADF emitters for OLED technologies. We have now amended the discussion for this part in the revised manuscript on page 17, lines 14-16: "Building upon our current study, we anticipate that such strategy could be applied to highly emissive narrowband multi-resonance TADF systems to fully harness the potential of these promising candidates for OLED applications." Minor issues: Page 4 Line 8: "…mechanism of such effect has yet been…" should be ""…mechanism of such effect has not yet been…" Page 4 Line 11: The author may consider revising "full potential of fully deuterated materials" to "capacity of fully deuterated materials". Page 4 Line 19: "a series of deuterated isomers" They are not isomers, they are isotopologues. Please double check it throughout the entire manuscript.
Page 7, lines 21-22: "…the strengthening of multiple C-D bonds in the compounds is suggested to prevent most of the proton abstraction reaction and subsequent radical formations." Page 7, lines 7-8: "It should be noted that the addition of ZPE values of DPA and BO precursor compounds are not equal to the ZPE of the final compounds." Page 12, line 15: "…and a single spin signal with g value…"

Reviewer #2 (Remarks to the Author):
Tang and co-workers reported a comprehensive study on the subatomic modified thermally Response: We appreciate the reviewer for the recognition of our work and the valuable comments.
1. The delta Est of these compounds in the solid-state thin films should be determined by PL transient emission spectra, instead of the steady state emission spectra as shown in the Figure S35.
Response: As suggested by the reviewer, we have determined the ∆EST of the compounds in the solid-state thin films using PL transient emission spectra shown in Figure S38, which also includes the steady-state emission spectra. In the 10% DPEPO thin film at 300 K and 77 K, the DPA-BO compounds exhibit a broad, structureless fluorescence and phosphorescence emission band, with emission maxima observed at 450 nm and 454 nm, respectively. To ensure accuracy and eliminate potential errors arising from emission originating from the S1 state, emission spectra for all compounds were also obtained with a gate delay time of 100 µs, and no significant changes in the emission maxima were recorded. We then determined the ∆EST of the compounds in the solid-state thin films by the emission maximum derived from the emission spectra obtained at 300 K and delayed 77 K. The estimated ∆EST values for all compounds are found to be approximately 0.08 eV, indicating an insignificant change of the ∆EST values determined either by the steady or time-delayed emission spectra. We have now amended the discussion for this part in the revised manuscript on page 9, lines 1-4: "the PL transient emission spectra recorded at low temperature and the steady-state emission spectra at room temperature, representing the phosphorescence and fluorescence emission of the compounds doped at 10 wt% in DPEPO (Figure S38), where an estimated ∆E(S1−T1) of ca. 0.08 eV is found for all compounds." 2. The commercial source or the preparation of the deuterated chemicals should be provided.
Response: Thank you for the reminder. We apologize for the missing information and have now provided the source of the commercially available deuterated chemicals in the "Methods" section in the main text on page 18, lines 1-7 as follows: "Materials. 2,5-dibromo-1,3-difluorobenzene, potassium carbonate, sodium tert-butoxide and tris(dibenzylideneacetone)dipalladium were purchased from Leyan. Phenol, Aniline-d5 and bromobenzene-d5 were purchased from Aladdin. Phenol-d5, diphenylamine and boron tribromide  were purchased from Sigma-Aldrich. Tri-tert-butylphosphine was purchased from Adamas. All solvents were purchased from General-Reagent. All commercially purchased chemicals were used without any further purification." 3. Please double check the figure 3c with the table 1, the delayed lifetime seems to be inconsistent.  We have now updated the figure caption of the corresponding results in Figure S43 as "PL spectra of ( On the other hand, we have also conducted the photolysis study in doped mCBP film (Figures 4c   and 4d), which allow us to compare the PL and operation stability tests of our sensitized OLED,

Response
where DPA-BO, BN3 and mCBP serve as the sensitizer for multi-resonance TADF, the guest material, and the host material, respectively. We observe a decrease in the excited state lifetime of BN3 from 224 µs to 196 µs (Figure S46c), indicating an efficient radiative decay from the S1 state of BN3 followed by complete FRET. The reduction in the density of triplet excitons serves to suppress bimolecular triplet-triplet annihilation and triplet-polaron annihilation, which can otherwise lead to undesired exciton fusion processes, such as high-energy polaron formation, ultimately resulting in emitter degradation. These findings align with the results of our OLED operation test, as presented in Table 3       Von represents turn-on voltage.

d)
CIE coordinates are taken at luminance of 100 cd m −2 .

e)
EQEmax represents maximum external quantum efficiency.

g)
CE represents maximum current efficiency.

h)
PE represents maximum power efficiency.

i)
Operational lifetime projected at 1000 cd m -2 .

Can the authors provided explanations for the limited device performances in non-sensitized
devices and sensitized device 1 and device 2.
Response: Thank you for the reviewer's question. As shown in the Figure 6b, the device characteristics of vacuum-deposited TADF-sensitized OLEDs with t-Bu-v-DABNA showed an emission band peaking at 461 nm, with CIE coordinates of (0.13, 0.09). According to a recent study (Adv. Funct. Mater. 32, 2110356 (2022)), t-Bu-ν-DABNA emits at 2.60 eV, with a delayed excited lifetime of 2.93 µs and a kRISC value of 2.54 × 10 5 s −1 . This relatively long excited state lifetime could induce the formation of high-energy excitons through processes such as bimolecular triplet-triplet annihilation or triplet-polaron annihilation. These processes may contribute to the degradation of the emitters. It is worth noting that the OLED stack employed in our study may not be optimally suited for saturated blue OLED, for which the availability of suitable host material is limited. As a result, for the vacuum-deposited devices with DPA-BO sensitizing t-Bu-v-DABNA, there could be possible energy mismatch of the host materials. In addition, and high emission energy of t-Bu-v-DABNA could lead to the formation of high energy exciton (> 5 eV), which is known to degrade the emissive layer rapidly and thus limiting the device operation lifetime of sensitized devices 1 and 2. As for non-sensitized device, the relatively long excited state lifetime of BN3 (224 µs) is undesirable for achieving a reasonable device stability, which also causes the limited device performance.
6. In Figure S38, why the intensity drop for DPA-BO is smaller than d-DPA-BO, please double check the data.
Response: Thank you for pointing out this issue. To further validate the intensity drop in the UV-vis absorption spectra of the compounds, we repeated the measurements multiple times, maintaining consistent concentration of the solutions for all the samples to ensure accuracy in the measurements. We re-examined the UV-vis spectra of the compounds and incorporated the updated data in Figure S41 of the revised manuscript. The revised figure illustrates the trends of UV-vis absorbance intensities for the series of compounds, which are in good agreement with their degree of deuteration. 7. It is suggested to change light green lines to a darker color for clarity in the whole manuscript.
Response: As suggested, a darker green colour has been adopted in both the revised manuscript and the supplementary information for clarity.

Reviewer #3 (Remarks to the Author):
Tang and co-workers reported a series of TADF emitters upon systematic deuteration. The authors clearly express their strategy for prolonging the device operational stability in the introduction section. They also investigated the effect of deuteration on photophysical behavior, intrinsic stability, and device operational stability. Overall, this manuscript is well-written and recommended for publication in Nature Communications after a minor revision as follows.
Response: We sincerely thank the reviewer for his/her positive feedback on our manuscript.  1. In the introduction section, on page 3, line 32, the authors mentioned that the increased PLQY of the near infra-red emitters is due to the lowering of the non-radiative decay rate (knr) and the reorganization energy. Did the authors investigate the effect of deuteration on knr, and even the

Huang-Rhys factor?
Response: We appreciate the insightful feedback provided by the reviewer, which prompted us to perform additional calculations on the kr, knr and the Huang-Rhys factors (Sj). We found that there were no obvious trends in the displacement (∆Q), Sj and reorganization energies (λj) with respect to the deuteration of the compounds, and the kr (1.18 ´ 10 8 s -1 ) and knr (ca. 1.4 ´ 10 11 ) remained mostly unchanged upon deuteration. The contribution of normal mode vibration to the reorganization energy is plotted in Figure S32 and the selected normal modes are illustrated in Figures S33 and S34. These results clearly demonstrate that the lj value is primarily influenced by low-frequency vibrational modes below 50 cm -1 , which correspond predominantly to the rotation of the phenyl rings in both the DPA and BO moieties.
We have now added the following discussion in the main text on page 7, lines 25-35, and page 8, lines 1-2: "In addition, the effects of deuteration of the compounds on their Huang-Rhys factor (Sj), reorganization energy (λj), radiative (kr) and nonradiative (knr) decay rate constants have been investigated. Shown in Figure S32 are the plots of the computed λj as a function of normal mode wavenumbers for S1-S0. It is obvious that the λj is contributed predominantly by the low-frequency vibrational modes under 50 cm -1 . These low-frequency vibrational modes are illustrated in Figures S33 and S34, and they are mainly the rotation of the phenyl rings in both the DPA and BO moieties, which contribute predominantly to the distortion of the S1 geometry from the S0 geometry.
The displacement (∆Q), Huang-Rhys factors (Sj) and reorganization energies of selected normal modes of the compounds are summarized in Tables S8 and S9, and the computed kr and knr values are summarized in Table S10. There is no obvious trend in ∆Q, Sj and λj upon the deuteration of the compounds, and the kr (1.18 × 10 8 s -1 ) and knr (ca. 1.4 × 10 11 s -1 ) are nearly the same upon deuteration. Therefore, deuteration does not have significant effect on kr and knr in this case, resemblance to our photophysical studies." Table S8.

DPA-BO d-DPA-BO
wj ( Figure S33. Illustration of selected normal modes contributing to large reorganization energies for the S0 state. Figure S34. Illustration of selected normal modes contributing to large reorganization energies for the S1 state.
2. Besides deuterium, do the authors think of any isotope of other elements that can be applied in their future studies?
Response: In addition to deuterium, researchers can explore other stable isotopes of elements such as carbon-13, oxygen-18 and nitrogen-15 for future investigations in organic electronics and the related fields. These isotopes might be used to modify organic materials or molecules in diverse ways, enabling the enhancement of specific material properties and a deeper understanding of the corresponding behaviour. Building upon the present study, our investigation into new classes of TADF and MRTADF emitters with phenol-C13 and carbazole-N15 derivatives as starting materials is currently in good progress. These findings will be reported in due course.
3. On page 7, line 5, regarding the statement "suggesting the need of a higher activation energy (Ea) for C−D bond breaking in the deuterated compounds". In fact, the lowering of the ZPE can be observed in both the reactants and products, and this nearly cancels out for the BDE. However, the effect would be more pronounced for barriers as the transition states often have weak vibrations with low frequencies. As a result, investigating the transition state and the corresponding activation energy of the C-H, C-D and C-N bond dissociations would help further understand the reason for the OLED stability.

Response:
We appreciate the reviewer's suggestion. We have now optimized the transition states for the C-N bond dissociation of both DPA-BO and d-DPA-d-BO. Subsequently, we calculated the corresponding activation enthalpies for these reactions. The obtained results have now been incorporated into the updated version of Table S7. The calculated activation enthalpies for the C-N bond dissociation are approximately 0.90 eV, and there is minimal difference observed between the deuterated and non-deuterated compounds. In Qiao and coworkers' study (Nat. Commun. 14, 3927 (2023)), the BDE at the T1 state was found to be linearly correlated with logarithm of device lifetime. Nevertheless, it is suggested that the BDE is more sensitive to the molecular structure than to the deuteration. Therefore, it is plausible that the BDEs found in our study do not show significant difference upon deuteration. Though the deuteration of DPA-BO does not strengthen the C-N bond, nor does it influence the rate of C-N bond dissociation, the strengthening of multiple C-D bonds in the compounds is suggested to prohibit most of the proton abstraction reactions and subsequent radical formations. In an attempt to optimize the transition states for the C−H (or C−D) bond dissociation, we encountered convergence issues, likewise, the typical case of H2 dissociation, which highlights the limitations of density functional approximations. These limitations can be circumvented by performing calculations on the C-H or C-D bond activation reactions of the compounds by other molecules or radicals present in the system. Such calculations are undergoing in another TADF system and will be reported in due course. We delete the inappropriate statement "suggesting the need of a higher activation energy (Ea) for C−D bond breaking in the deuterated compounds". In addition, we rewrite the paragraph after "The ZPE of these compounds and DPA and BO precursor compounds are listed in Table S4, whereas the BDE of the C−N bond and the C−H (or C−D) bond are summarized in Tables S5 and S6." to enhance the clarity.
We have now revised the manuscript to clarify this point on page 7, lines 5-22: "Regarding the C-N bond dissociation, the ZPE decreases from 262.9 kcal mol -1 to 226.1 kcal mol -1 upon increasing the degree of deuteration in general (Table S4) , the additive effect should be appreciable, and hence the proton abstraction reaction is thermodynamically less favourable and subsequent formation of high molecular adducts is probably suppressed. 12,32 In short, upon deuteration of DPA-BO, though the C-N bond is slightly weakened, the strengthening of multiple C-D bonds in the compounds is suggested to prevent most of the proton abstraction reaction and subsequent radical formations. With these calculated results, we anticipate that the higher degree of deuteration on both donor and acceptor units should exert a more prominent effect on the device stability." 4. On page 25 of the supporting information regarding Figures S29 and S30. For the S1 state of DPA-BO, the LSOMO is on the DPA moiety, whereas the HSOMO is on the BO moiety. For the T1 state of DPA-BO, the LSOMO is on the BO moiety, while the HSOMO is on the DPA moiety.
Could the authors please check if they are correct?

Response:
We apologize for the misleading information in the figures. We have checked and confirmed that, for the T1 state of DPA-BO, the LSOMO is localized on the DPA moiety whereas the HSOMO is localized on the BO moiety. Figure S30 has been revised with the correct spatial plots of HSOMO and LSOMO as shown below. Figure S30. Spatial plots (isovalue = 0.03) of selected molecular orbitals of DPA-BO at the optimized T1 state geometry.
5. On page 7, line 17, the authors mentioned, "the proton abstraction reaction or high molecular adducts formation is probably less favourable". Could the authors describe more specifically the possible pathways of the proton abstraction reaction and the subsequent degradation of the compounds?
Response: Thank you for your comments. One of the possible pathways of the proton abstraction reaction is shown in the following Scheme R1, where the C-N bond would undergo cleavage, resulting in the formation of DPA and BO radicals that subsequently initiate further reaction by attacking neighbouring DPA-BO molecules. According to the work by Lee and co-workers in 2016(Adv. Opt. Mater. 4, 1281(2016) and other relevant studies, the C-N bond is generally acknowledged as the weakest chemical bond within the backbone structure of host materials. The low BDE of the C-N bond can lead to reduced stability and shorter lifetime of blue TADF-OLED devices (Nat. Commun. 14, 3927 (2023)). In the case of DPA-BO, the weak C-N bonds may be prone to breaking due to the high excitation energy and prolonged triplet-state lifetime. Consequently, the resulting DPA and BO radicals can engage in reactions such as proton abstraction with neighbouring DPA-BO molecules, leading to formation of additional radicals. When considering deuterated DPA-BO molecules, the abstraction of deuterium is expected to be less favourable due to the primary isotopic effect as reported in other organic reactions. Based on this understanding, we propose that the introduction of deuterium into the compounds would enhance their resistance to such reactions, thereby improving the stability of OLEDs, as demonstrated in our study.
More comprehensive experimental and computational investigations, including mass spectrometry analysis of the generated products, as well as mechanistic and kinetic studies of both deuterated and non-deuterated DPA-BO, are necessary to enhance our understanding of the underlying mechanisms, with respect to thermodynamic parameters and the rate coefficients of the C-N bond cleavage and the subsequent proton abstraction processes. As such investigations are ongoing and would be beyond scope of the current study, we have decided not to include the proposed mechanism in the revised manuscript or the supplementary information. Nevertheless, we are revising the relevant text in the main body of the manuscript to provide additional insights and discussion on the topic.

Scheme R1.
Proposed reaction pathways of the diarylamine radical towards DPA-BO compound.
6. On page 8, line 17, the authors mentioned "indicating the presence of strong intermolecular interactions in solid state……". What kind are those possible intermolecular interactions do the authors suggest?
Response: We suggest that the intermolecular interactions to be the non-covalent interactions such as π-π interaction, which is known to cause a red shift in photoluminescence spectra, in particular in the solid-state thin film. The photoluminescence spectra of the tested TADF molecules in toluene and in doped films are shown in Figure 3a and Figure S36, respectively. Figure 3a shows no shift in emission energy, while the luminescence peak redshifts from 429 nm to 452 nm as the DPA-BO concentration increased from 5 to 20 wt% in DPEPO films ( Figure S36). These results indicate the presence of π-π interactions assumed the ground state dipole of this series of compound is of similar to that of DPEPO. To further investigate the possible intermolecular interactions in the solid-state thin film, we have additionally performed DFT calculations on the dimer form of DPA-BO at the at the M06/6-31G(d,p) level. The distance between the two planes of the BO moieties is 3.26 Å, which is smaller than the separation between the π-π planes (ca. 3.35 Å) ( Figure R3 and Table R1). Therefore, this additional computational result further confirms the presence of π-π interactions in the solid-state thin film.